Conventional butter products contain about 80% fat, 15% water and 1.5% salt, and a mixture of minor amounts of other milk components that are carried over into the butter during the churning process. The problem with conventional butter products is essentially two-fold. First, these products are highly concentrated forms of valuable butterfats, that in some cases are in short supply and in others can find productive applications other than in the form of conventional butter products. Secondly, while highly nutritional, butterfat belongs to a class of foods which are often in excess, at least in North American diets. As a consequence of these factors there has been much interest in developing butterfat or butterfat substitute products which are functionally fungible with conventional butter products.
Much of the technological focus on low-fat butter or butter substitutes has been on attempts to blend ever larger proportions of water into fat/water mixtures. The addition of water to fats in these mixtures inherently raises issues of emulsion instability, with it's associated problems becoming progressively more exacerbated in proportion to the increasing amounts of water that are incorporated into the mixtures.
A case in point concerns attempts at forcing large amounts (i.e. about 60%) of water into a 40% fat envelope, to produce a low-fat form of the generally preferred water in oil type emulsion. (This type of emulsion is preferred over oil in water emulsions because the kinesthetic properties of the latter are markedly dissimilar to butters corresponding organoleptic properties. In addition, edible oil in water emulsions have notoriously poor microbiological stability. Moreover, the physical stability issues at low-fat concentrations in oil in water emulsions are fundamentally different from the physical stability of water in oil emulsions, and the two are not at all comparable in their behavior in this regard). Although there is a substantial body of literature documenting the extensive efforts that have been expended on dealing with these instability problems, relatively little is known beyond the strictly empirical solutions that have been offered to date. According to the Encyclopedia of Chemical Technology: "Emulsion technology is at present based on a trial and error experience and a quasi- logical extension of that experience."
In general when the relative proportions of the continuous phase and the discontinuous phase reach certain critical concentrations, the emulsion tends to destabilize. This is central to the problem faced in low-fat butter products where the continuous fat phase is stretched to the limits of it's ability to contain the relatively high proportions of water that are typical of these products.
Instability can manifest either as phase separation or phase reversal. When the relative concentrations of the phases approach the above-mentioned critical conditions, it is necessary for other characteristics of the emulsion to augment the stability of the emulsion in order to meet commercial product specifications. In the case of low-fat butter, those specifications call for a water in fat phase relationship which resists phase separation (i.e. weeping, wheying-off, or bleeding). In general, the greater the resistance these products have to phase separation the better, and the more closely the product will emulate butter.
The problem of instability has been addressed chemically. Three approaches have been tried: 1) alteration of the fat components through addition of other fats and/or refinement of certain butterfat species; 2) addition of emulsifying agents, from both natural and commercial sources; 3) addition of stabilizing agents such as gums and gelling agents. All these approaches retard, to greater or lesser degrees, the coalescence of the discontinuous phase of the emulsion.
The alteration of the fat components is aimed at increasing the viscosity of the emulsions continuous phase, by increasing the degree of saturation, or the average molecular weight, or both, and hence increasing the average melting point temperature of the continuous phase. This solution has been employed commercially, but suffers from the fact that the changes in question change the plasticity of the fats and modify the kinesthetic properties of the resulting products. Other organoleptic properties may also be adversely affected, especially in the case where other fats such as vegetable fats are added. Additionally, market studies have shown a marked consumer resistance to the addition of non-dairy fats. On the other hand, refinement of dairy fat components is a very expensive alternative and has not been widely adopted.
The second of the above-mentioned approaches to stabilizing the emulsion chemically, involves the use of emulsifying agents. Broadly speaking, these agents include natural sources of emulsifying agents such, as for example, non-fat milk solids and buttermilk. Caution must be exercised in the use of these emulsifying agents since these are the same agents which normally support the oil in fat emulsions of milk and cream and might therefore increase the risk of phase reversal when used in water in fat emulsions. It is significant that buttermilk is separated out during churning of conventional butter products. Other natural emulsifiers include various soy fractions and the like, although theses suffer from the disadvantage of not being indigenous to dairy products, and hence do not enjoy the "natural" connotation. Commercial emulsifying agents include lecithin, various phospholipid preparations, surface active agents ("detergents") and distilled mono and diglycerides Strong commercial emulsifying agents, however, have the potential to actually destabilize the emulsion by reducing the apparent viscosity. Moreover, North American marketing research attributes much of the lack of commercial success of products containing such agents, to consumer attitudes towards the use of "chemical" additives in foods.
The third approach involves the use of stabilizers to increase the viscosity of the water phase. Water-soluble gums and gelatins are useful for this purpose. Again, the addition of these agents is not well received by consumers.
The instability of water in oil emulsions at high concentrations of water has also been addressed through mechanical emulsification treatments. In general, as the proportion of the dispersed phase realtive to the continuous phase increases, the viscosity of the emulsion also increases. In particular, when the volume of the dispersed phase exceeds the volume of the continuous phase, the emulsion particles become crowded and the "apparent" viscosity of the emulsion takes on a "structural" component over and above the viscosity contributed by the continuous phase. In order for this structural viscosity to manifest in emulsions with high concentrations of a dispersed phase, the particle size of the dispersed phase must be small enough to resist spontaneous coalescence and emulsion destabilization. Mechanical emulsification is essential for these purposes.
The churn is one of the many devices that has been employed to this end, even if perhaps for no better reason than it's long association with the butter industry. Bullock, J. Dairy Science, vol.52, no.5, 1969 found that serum and butter mixtures could be placed in plastic bags and tumbled in a churn. After one hour in the churn, these laboratory scale mixtures had formed water in oil emulsions with small, fairly well distributed water droplets. The action of the churn depends for its effect entirely upon so-called "turbulent mixing" in which turbulence and diffusion result in both particle size reduction and dispersion of the discontinuous phase. While perhaps suitable for the processing of low viscosity emulsions, such mixing is very inefficient when dealing with more viscous emulsions (i.e. where viscosity is high enough to negate mixing forces based of turbulence and diffusion alone). Note that a full hour was required to process even the very small test samples described in the Bullock reference.
A wide variety of emulsification equipment has been used with the objective of breaking up or dispersing the discrete liquid phase in the fat phase so that the particles of the dispersed phase in the resulting emulsion are small and uniform enough to prevent coalescence and consequential breakdown of the emulsion. This aspect of the emulsions stability is most significant during post-emulsification handling of the product, and remains so until fat crystallization and thixotropic aging of the emulsion brings the product's stability up to its full potential.
In general, emulsification equipment is divided into two general categories: propeller emulsifiers and turbine emulsifiers. The Hobart mixer described by Bullock et al falls into the former category, and although that particular model is no longer commercially available it is, according to the manufacturer, equivalent to Hobart Model A-200T 20 quart Mixer which operates a variety of agitators and beaters and the like, at speeds of between about 100 and 400 rpm. This mixer produces both dispersive and distributive forces, and relies primarily on turbulent mixing for this purpose. Stephan Machinery Corporation recommends the use of its models UMM/SK and TC/SK type mixers for low-fat spread applications. The UMM/SK and TC/SK type mixers are essentially multiple propeller type mixers having characteristics which to some degree minimize the limitations imposed by simple turbulent mixing through the introduction of larger mechanical shear components into the mixing process. See for example, U.S. Pat. No. 4,056,640.
Since turbulent mixing depends primarily on turbulence and diffusion, its usefulness as a standalone technique for producing fine emulsions is somewhat limited to the processing of low viscosity fluids, even though, in gross, the folding over of higher viscosity mixtures Can be adequate for some purposes, (as in the case of baking doughs, for example). Processes for producing fine emulsions of relatively high viscosity fluids, are known which involve the use of homogenizers and colloid mills. Both these devices have been classified as "modified" turbine emulsifiers, (see Encyclopedia of Chemical Technology, Volume 5, Page 705) and both are known to be capable of producing high levels of fluid shear, as reflected by the associated mechanical heat rise during processing.
In a homogenizer, emulsification is effected by forcing the two phases past a spring-seated valve. This is usually done at relatively high pressures of 500 to 3,000 psi. Emulsification occurs not only while the components pass under the valve seat but also when the emulsion impinges against the retaining wall that surrounds the valve. As a general rule, homogenizers usually give an emulsion of finer average particle size than colloid mills, although the particle size is not as uniform. Possibly this is a reflection of greater dispersive forces at play in the homogenizer, but that such processing is much more statistical (i.e. non-uniform). A mechanical temperature rise of about 10.degree. F. to 30.degree. F. (6.degree. C. to 17.degree. C.), is typical of homogenization processes, although depending on the type of supply pump that is used, this may run as high as between 50.degree. F. to 90.degree. F.
In low-fat butter processing, it is generally acknowledged that high pressure treatments have a disadvantageous, destabilizing effect on the emulsion. In addition, lack of uniform processing in homogenizers may leave a proportion of the dispersed phase in the form of particles which are large enough to act as or promote the formation of coalescence nuclei either during subsequent processing or in the final product. It is known, for example, that particles of different sizes coalesce more easily than do particles of the same size. Once such coalescence begins, it has the potential to destabilize significant amounts of the emulsion and result in weeping, etc.
Colloid mills on the other hand, produce the desired high degree of uniformity of particle size without necessarily engendering the kinds of operating pressures associated with homogenizers. South African patent application number 86/2344, for example, cites line pressures of between 80 and 116 psi, with pressure drops across the mechanical emulsifier in the range of 22 to 58 psi. This mechanical emulsifier is specially designed for the purpose of minimizing operating pressures. For these reasons, much interest has been shown in their application in the production of low-fat butter products.
Unfortunately, colloid mills in general are known to result in very high mechanical temperature rises, on the order of between 30.degree. F. to 140.degree. F. (17.degree. C. to 79.degree. C.). The resulting high processing temperatures are known to decrease the viscosity of the continuous phase of the emulsion and have an adverse effect on its stability. In addition, although the dispersive forces generated in these devices produce highly uniform particle sizes, they do not appear to produce corresponding levels of distributive forces. Without such distributive forces, inter-particle distances within the emulsion are not maximized and the more closely packed particles of the discontinuous phase will have an increased probability of initiating coalescence, manifesting in gross destabilization of the emulsion.
A series of South African patent applications assigned to Unilever, (including the above-mentioned South African patent application number 86/2344), deal with a process in which a thermoplastics extruder is utilized for the purpose of producing low-fat butters and the like. The device can be thought of as a modified colloid mill and is described in detail in U.S. Pat. No. 4,419,014.
The device is intended to produce a smooth streamlined flow with limited substrate exposure to simple shear across shear lands (for dispersive mixing) and laminar shear within the hemispherical cavities (to facilitate more uniform distributive mixing) over and above such turbulent mixing ("folding") as results when the substrate flow is repeatedly subdivided (across the shear lands) and recombined during its transit through the device. According to the literature, this arrangement is thought to reduce operating back-pressures, as already noted herein, as well as reduce mechanical temperature rise and improve uniformity of processing by reducing product back flow within the device.
Notwithstanding this purported reduction in simple shear and increased laminar shear (and better distributive mixing) and even the supposed reduction in processing delta-t and more uniform substrate treatment, the process in question still requires temperature processing control. The use of an integrated heat exchanger apparently results in a more uniformly dispersed emulsion, presumably because the fat phase is sufficiently viscous at the reduced processing temperatures to retard post emulsification coalescence of the dispersed phase. One of the Unilever patents discloses that such temperature control is essential to producing the homogeneity which is taught to be essential to that process. Temperature control therein is affected by maintaining stator surface temperatures of -20.degree. C., in order to keep the average delta-t of the substrate within the range of 2.degree. C. to 10.degree. C.
There are a number of problems associated with this approach. First of all, optimal fat crystallization is an extremely complex interrelationship between endo and exothermic reactions within the processing milieu. External temperature control alters the thermodynamics of such processes on the microstructural level even while attempting to compensate for the mechanical thermodynamic inputs in gross. This is a necessary consequence of heat transfer inertia into and within the substrate during processing even between the narrow annulus formed between the rotor and stator. This problem cannot be helped any by the fact that in some embodiments the stator bears the temperature controlled processing surface, even though the mechanical energy density is highest along the rotor/substrate interface adjacent which the highest substrate acceleration occurs. In any case, the microcrystalline structure of the fat phase does not appear to be stabilized as effectively as might be desired, through the use of such overt external cooling. Consequently, even though South African patent application number 86/2344 indicates that margarine products produced using this process can be packed as "cakes", there is no disclosure of any ability to print products based solely on butterfat emulsions containing large amounts of water, which is presumably a reflection of the latter emulsions (as produced in accordance with the application) inability to survive the rigors of the printing process.